TNF ligand-receptor interactions serve diverse functions in immune system regulation. Fas, a member of the TNF receptor family is an important regulator of lymphocyte homeostasis and can cause systemic autoimmunity when genetically mutated. Althuogh a number of signaling proteins mediating TNF receptor signals have been identified, the cell biology of receptor signaling: where signaling events take place within the cell and whether subcellular localization plays a role in regulating TNF receptor signaling, remains relatively unknown. Understanding of this process is important for devising better ways to modulate signaling through these receptors for therapeutic goals. In previous work, we established that TNF receptors must pre-associate into mutimeric complexes to signal efficiently. Preassociation of Fas and TNFR1 is mediated by an N-terminal pre-ligand association domain (PLAD). We are currently investigating whether adding bacterially synthesized PLAD domain fragments can bind to native receptors and inhibit their function. For the apoptosis-inducing receptor Fas (CD95, APO-1), we have more recently characterized much larger microscopically visible surface receptor clusters initiated by antibody or ligand that are dependent on the death domain and are required for caspase-8 activation and apoptosis signaling. Additionally, we have found a role for lipid raft microdomains in regulating the efficiency of Fas signaling.